Planet Profile

History of Jupiter

Jupiter (a.k.a. Jove; Greek
Zeus)
was the King of the Gods, the ruler
of Olympus and the patron of the Roman state.
Zeus was the son of Cronus (Saturn).

Jupiter is the fourth brightest
object in the sky (after the Sun, the
Moon and Venus).
It has been known since prehistoric times as a bright "wandering star".
But in 1610 when Galileo
first pointed a telescope at the sky he
discovered
Jupiter's four large moons
Io,
Europa,
Ganymede and
Callisto
(now known as the Galilean moons) and recorded their motions
back and forth around Jupiter. This was
the first discovery
of a center of motion not apparently centered on the Earth.
It was a major point in
favor of Copernicus's
heliocentric theory of the
motions of the planets (along with other new evidence from his telescope:
the phases of Venus
and the mountains on the Moon).
Galileo's outspoken support of the Copernican theory got him in trouble with
the Inquisition.
Today anyone can repeat Galileo's observations (without fear of retribution :-)
using binoculars or an inexpensive telescope.

The gas planets
do not have solid surfaces, their gaseous material simply gets
denser with depth
(the radii and diameters quoted for the planets are for levels
corresponding to a pressure of 1
atmosphere).
What we see when looking at these planets is the tops of clouds
high in their atmospheres (slightly above the 1 atmosphere level).

Jupiter is about 90%
hydrogen
and 10% helium
(by numbers of atoms, 75/25% by mass)
with traces of methane,
water, ammonia and "rock".
This is very close to the composition of the primordial
Solar Nebula from
which the entire solar system was formed. Saturn
has a similar composition,
but Uranus and
Neptune have much less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas planets)
is highly indirect and likely to remain so for some time.
(The data from
Galileo's atmospheric
probe
goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to
something like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in
the form of liquid metallic hydrogen.
This exotic form of the most common of
elements is possible only at pressures exceeding 4 million
bars, as is the case in the interior of
Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized
protons and electrons (like the interior of the Sun but at a far lower
temperature). At the temperature and pressure of Jupiter's interior
hydrogen is a liquid, not a gas.
It is an electrical conductor and the source of
Jupiter's magnetic field.
This layer probably also contains some helium
and traces of various "ices".

The outermost layer is composed primarily of ordinary molecular hydrogen
and helium which
is liquid in the interior and gaseous further out. The atmosphere we
see is just the very top of this deep layer.
Water, carbon dioxide, methane and other simple molecules
are also present in tiny amounts.

Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.

Three distinct layers of clouds
are believed to exist consisting of ammonia ice, ammonium
hydrosulfide and a mixture of ice and water. However, the
preliminary
results
from the Galileo probe show only faint indications of clouds (one instrument
seems to have detected the topmost layer while another may have
seen the second).
But the probe's entry point (left) was unusual --
Earth-based telescopic observations and more
recentobservations
by the Galileo orbiter suggest that the probe entry site
may well have been one of the warmest and least cloudy areas on
Jupiter at that time.

Data
from the Galileo atmospheric probe also indicate that there is
much less water than expected. The expectation was that Jupiter's
atmosphere would contain about twice the amount of oxygen (combined with
the abundant hydrogen to make water) as the Sun. But it now appears that
the actual concentration much less than the Sun's.
Also surprising was the high temperature and
density of the uppermost parts of the atmosphere.

Jupiter and the other gas planets have
high velocity winds which are confined in wide bands of
latitude. The winds blow in opposite directions in adjacent bands.
Slight chemical and temperature differences between these bands
are responsible for the colored
bands that dominate the planet's appearance.
The light colored bands are called zones; the dark ones belts.
The bands have been known
for some time on Jupiter, but the
complex vortices in the boundary regions between the bands were
first seen by Voyager.
The data from the Galileo probe indicate that the winds are even
faster than expected (more than 400 mph) and extend down into as far as
the probe was able to observe;
they may extend down thousands of kilometers into the interior.
Jupiter's atmosphere was also found to be quite turbulent.
This indicates that Jupiter's winds are driven in large part by
its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter's clouds are probably the result of
subtle chemical reactions of the trace elements in Jupiter's
atmosphere, perhaps involving sulfur whose compounds take on a wide
variety of colors, but the details are unknown.

The colors correlate with the cloud's altitude: blue lowest, followed
by browns and whites, with reds highest. Sometimes we see the lower
layers through holes in the upper ones.

The Great Red Spot (GRS)
has been seen by Earthly observers for more than 300 years
(its discovery is usually attributed to Cassini,
or Robert Hooke in the 17th century).
The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths.
Other smaller but similar spots have been known for
decades.
Infrared observations and the direction of its rotation indicate that
the GRS is a high-pressure region whose cloud tops are significantly
higher and colder than the surrounding regions.
Similar structures have been seen on Saturn and
Neptune.
It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives
from the Sun.
The interior of Jupiter is
hot: the core is probably about 20,000 K.
The heat is generated
by the Kelvin-Helmholtz mechanism, the slow gravitational
compression of the planet. (Jupiter does NOT produce energy by
nuclear fusion as in the Sun;
it is much too small and hence its interior
is too cool to ignite nuclear reactions.) This interior heat
probably causes convection
deep within Jupiter's liquid layers and is probably
responsible for the complex motions we see in the cloud tops.
Saturn and Neptune are similar to Jupiter in this respect, but
oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more
material were to be added, it would be compressed by gravity such that the
overall radius would increase only slightly. A star can be larger only because
of its internal (nuclear) heat source.
(But Jupiter would have to be at least 80 times more massive to become a star.)

Jupiter has a huge magnetic field, much stronger than Earth's.
Its magnetosphere extends
more than 650 million km (past the orbit of Saturn!). (Note that
Jupiter's magnetosphere is far from spherical -- it extends "only" a few
million kilometers in the direction toward the Sun.)
Jupiter's moons therefore lie within its magnetosphere, a fact which
may partially explain some of the activity on Io.
Unfortunately for future space travelers and of real concern to the designers
of the Voyager and Galileo spacecraft, the environment near Jupiter
contains high levels of energetic particles trapped by Jupiter's magnetic
field. This "radiation" is similar to, but much more intense than, that
found within Earth's Van Allen belts.
It would be immediately fatal to an unprotected human being.
The Galileo atmospheric probe
discovered a new intense radiation belt
between Jupiter's ring and the uppermost atmospheric layers.
This new belt is approximately 10
times as strong as Earth's Van Allen radiation belts.
Surprisingly, this new
belt was also found to contain high energy helium ions of unknown origin.

Jupiter has rings like Saturn's, but much fainter and
smaller (right). They were totally
unexpected and were only discovered when two of the Voyager 1
scientists insisted that after traveling 1 billion km it was at
least worth a quick
look to see if any rings might be present. Everyone else thought that the
chance
of finding anything was nil, but there they were. It was a major coup.
They have since been
imaged
in the infra-red from ground-based observatories and by
Galileo.

Unlike Saturn's, Jupiter's rings are dark
(albedo about .05). They're probably
composed of very small grains of rocky material.
Unlike Saturn's rings, they seem to contain no ice.

Particles in Jupiter's rings probably don't stay there for long (due
to atmospheric and magnetic drag).
The Galileo spacecraft found clear evidence that the rings are continuously resupplied by
dust formed by
micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field.
The inner halo ring is broadened by interactions with Jupiter's magnetic field.

In July 1994, Comet Shoemaker-Levy 9 collided with
Jupiter with spectacular results (left). The effects were clearly visible even with
amateur telescopes. The debris from the
collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky
(it is second only to Venus, which is seldom visible in a dark sky).
The four Galilean moons are easily visible with binoculars;
a few bands and the Great Red Spot can be seen with a small astronomical
telescope.
There are several Web sites
that show the current position of Jupiter (and the other planets) in the sky.
More detailed and customized charts can be created with a
planetarium program.

Jupiter's Satellites

Jupiter has 79 known satellites : the four large
Galilean
moons plus many more small ones some of which have not yet been named:

Jupiter is very gradually slowing
down due to the tidal drag produced by the Galilean satellites.
Also, the same tidal forces are changing the orbits of the moons,
very slowly forcing them farther from Jupiter.

Io, Europa and Ganymede
are locked together in a 1:2:4
orbital resonance
and their orbits evolve together. Callisto is
almost part of this as well. In a few hundred million years,
Callisto will be locked in too, orbiting at exactly twice the
period of Ganymede (eight times the period of Io).

Jupiter's satellites are named for other figures in the life of Zeus (mostly
his numerous lovers).

Open Issues

Galileo's atmospheric probe provides
our first direct measurements of Jupiter's atmosphere, our first real
data about the chemistry of a gas planet. The initial data indicate a
major new mystery -- why is there so little water in Jupiter's atmosphere?
There is a building
consensus
that the probe encountered an unusually dry
area but more details are needed.

Just how deep into the interior do the zonal winds extend?
What mechanism drives them?

Why is the GRS so persistent? There are actually several theoretical models
that seem to work. We need more data to decide between them.

How can we get more direct information about the interior?
Liquid metallic hydrogen has been
produced in a lab
at Lawrence Livermore National Laboratory but much about its
properties is still unknown.